1. Field of the Invention
The present invention relates to an optical system that uses an optical material having extraordinary partial dispersion and is suitable for, for instance, a photographing optical system of a silver-halide film camera, a video camera, a digital still camera, or the like or a projection optical system of a liquid crystal projector or the like.
2. Related Background Art
It is required that a digital optical device, such as a digital camera, have a higher pixel density and provide higher image quality and that an optical system used therein have higher optical performance.
In general, an optical system for image pickup or projection has a tendency that as its lens entire length is reduced, chromatic aberrations, such as an axial chromatic aberration (longitudinal chromatic aberration) and a chromatic aberration of magnification (lateral chromatic aberration), out of various aberrations are increased and optical performance is lowered. As a method for suppressing the occurrence of such chromatic aberrations, an optical system where achromatization has been achieved using an extraordinary partial dispersion material Japanese Patent Application Laid-open No. 2000-267005 and Japanese Patent Application Laid-open No. 2001-194590 (corresponding to U.S. Pat. No. 6,404,561 B) and an optical system where achromatization has been achieved using a diffractive optical element having a diffractive action (Japanese Patent Application Laid-open No. H10-115777 (corresponding to U.S. Pat. No. 5,978,158 B) and Japanese Patent Application Laid-open No. H11-064726 (corresponding to U.S. Pat. No. 5,923,479 B) are generally well known.
Among those optical systems, the optical system where correction of chromatic aberrations is performed using the diffractive optical element utilizes a physical phenomenon where an output manner of the chromatic aberrations with respect to a light beam having a certain reference wavelength at a refractive surface in the optical system becomes opposite to that at a diffractive surface. This means that although ordinary optical glass has a positive dispersion characteristic, the diffractive optical element has a negative dispersion characteristic (νd=−3.453). In addition, the diffractive optical element has a strong extraordinary dispersibility (θgF=0.2956) and also has a characteristic where it is also possible to have an aspherical function by changing the periodical structure of the diffractive optical element. From above, the diffractive optical element is capable of achieving two profound effects that are a chromatic aberration correction effect and an aspherical effect utilizing the negative dispersion characteristic and the strong extraordinary dispersibility. Thus it can be expected that the optical performance will be significantly improved with the diffractive optical element. In addition, since the diffractive optical element is small in size, it is possible to extremely reduce its spatial occupancy, so that the optical system has features that it is easy to achieve weight reduction and miniaturization thereof.
Also, it is known that a liquid material that exhibits a relatively high dispersion and relatively extraordinary partial dispersion characteristics has a chromatic aberration correction action similar to that of the diffractive optical element and an achromatic optical system using the liquid material is proposed (U.S. Pat. No. 4,913,535 B).
In general, when correcting the chromatic aberrations of the optical system by means of the extraordinary partial dispersion material, this results in a tendency that the number of lenses of the optical system is increased and the entire optical length (distance from the first lens surface to an image plane) is increased. Also, there is another problem that extraordinary partial dispersion glass, such as fluorite, is very expensive and has a relatively large specific gravity as compared with another low dispersion glass that does not have extraordinary partial dispersion, which leads to an increase of the entire weight of the lens system (for instance, the specific gravities of fluorite and FK01 are respectively 3.18 and 3.63 but the specific gravities of FK5 and BK7 with small extraordinary partial dispersibilities are respectively 2.46 and 2.52). Further, there is still another problem that the surfaces of the extraordinary partial dispersion glass are relatively easy to be damaged and FK01 or the like is easily cracked in response to a sharp temperature change when the diameter of aperture is increased.
On the other hand, the diffractive optical element has a sufficient chromatic aberration correction action but has a problem that diffracted light with unnecessary diffraction-order light other than diffracted light with a design diffraction-order that is actually used becomes chromatic flare light and deteriorates imaging performance. A system is also known in which the unnecessary diffracted light is greatly reduced by concentrating energy to the design diffraction-order light by means of a so-called lamination-type diffraction grating where multiple blazed diffraction gratings have been laminated in an optical axis direction, but there still remains a problem that when photographing a highly bright object, diffraction flare appears.
Also, as a method of manufacturing the diffractive optical element, a method is known with which an ultraviolet curing resin or the like is molding using a die. Since the diffraction efficiency of the diffraction optical element is extremely sensitive to the manufacturing accuracy thereof, extremely high die accuracy and molding accuracy are required so that there is a problem that manufacturing cost of the diffractive optical element is high.
The material disclosed in U.S. Pat. No. 4,913,535 B is liquid, so a structure for sealing it is required and it is impossible to say that manufacturing is easy. Also, there is another problem that characteristics, such as a refractive index and a dispersion characteristic, depend on temperature and it is not necessarily possible to say that environmental resistance is sufficient. Further, the Abbe number is relatively large, the extraordinary partial dispersibility is relatively small, and it is impossible to obtain an interface with the air, so it is difficult to obtain a sufficient chromatic aberration correction action.